The present invention relates to inkjet printing devices, and particularly although not exclusively to a method and apparatus for servicing a pen when mounted in a printing device.
Inkjet printing mechanisms may be used in a variety of different printing devices, such as plotters, facsimile machines and inkjet printers, collectively called in the following as printers, to print images using a colorant, referred to generally herein as xe2x80x9cink.xe2x80x9d These inkjet printing mechanisms use inkjet cartridges, often called xe2x80x9cpens,xe2x80x9d to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as xe2x80x9coff-axisxe2x80x9d systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located xe2x80x9coff-axisxe2x80x9d from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a xe2x80x9cpen.xe2x80x9d
Each pen has a printhead that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The height of this band is known as the xe2x80x9cswath heightxe2x80x9d of the pen, the maximum pattern of ink which can be laid down in a single pass.
The orifice plate of the printhead, tends to pick up contaminants, such as paper dust, and the like, during the printing process. Such contaminants adhere to the orifice plate either because of the presence of ink on the printhead, or because of electrostatic charges. In addition, excess dried ink can accumulate around the printhead. The accumulation of either ink or other contaminants can impair the quality of the output by interfering with the proper application of ink to the printing medium. In addition, if colour pens are used, each printhead may have different nozzles which each expel different colours. If ink accumulates on the orifice plate, mixing of different coloured inks (cross-contamination) can result during use. If colours are mixed on the orifice plate, the quality of the resulting printed product can be affected. For these reasons, it is desirable to clear the printhead orifice plate of such contaminants and ink on a routine basis to prevent the build up thereof. Furthermore, the nozzles of an ink-jet printer can clog, particularly if the pens are left uncapped in an office environment.
In an off-axis pen, life goal is on the order of 40 times greater than a conventional non off-axis system, e.g. the printhead cartridges available in DesignJet(copyright) 750C color printers, produced by Hewlett-Packard Company, of Palo Alto, Calif., the present assignee. Living longer and firing more drops of ink means that there are greater probability that the printer print quality degrade and/or deviate along life. This requires finding better ways to keep functional and stable our printheads during long periods and large volumes of ink fired.
In U.S. Pat. No. 5,455,608 it is described how a printer may adjust servicing of the pen based on the result of the current drop detection step only. Before starting a plot these printers perform a drop detection on all the pens to detect if there are any non-firing nozzles (xe2x80x9cnozzles outxe2x80x9d). If a single nozzle out is detected in a pen, the printer triggers a so called automatic recovery servicing process for servicing the malfunctioning pen to recover the malfunctioning nozzle(s).
This process includes a sequence of 3 nozzle servicing or clearing procedures of increasing severity which are performed in sequence so long as some of the nozzles of the printhead fail to fire ink drops pursuant to ink firing pulses provided to the printhead or until all of the procedures have been performed.
At the end of each of these procedures a new drop detection is performed on the pen, to verify if the pen is fully recovered. If, according to the current result of the drop detection, it is not, the subsequent servicing procedure is performed. If, at the end of the 3 functions, the pen is still not fully recovered (i.e. at least one nozzles is still out) the user is reported to replace the pen or to disable the nozzle check. One big drawback of this system when implemented, e.g. as in DesignJet (copyright) 750 C printers, is that if the printer is not able to fully recover the failing nozzles or there are some unstable nozzles, the system will remain in this recovery servicing mode until the decease of the printhead, being forced, by the permanent nozzle out, to run this process at the beginning of each plot. This usually leads to either an unacceptable loss of throughput and printer productivity (because the printer stops and waits for an answer, the automatic recovery process is very time consuming, and causes a big waste of ink particularly when running the priming functions) or to excessive printhead replace or continue messages that users disable nozzle check via front panel, causing throughput losses.
With reference to the present application with the term plot it is identified any kind and size of printed output of the printer, seen by the printer as a single job. The plot could then identifies a CDA image or a graphic image like a photo or any other kind of print.
In order to maintain the quality of the printed output of the printer device it is important to improve the certainty that each instruction to the printhead to produce an ink drop from a nozzle of the plurality of nozzles does will produce such an ink drop (i.e. good servicing of the printhead and replacing nozzles out with working nozzles in performing error hiding).
The specific embodiments and methods according to the present invention aim to improve the servicing process without affecting the printing rate of such devices and thereby improving printing quality and the functional lifetime of the plurality of nozzles.
According to an aspect of the present invention, there is provided a method of servicing a pen comprising a printhead, having a plurality of nozzles, mounted in a inkjet printing device for printing plots, comprising a servicing area and a drop detector comprising the following steps (a) performing a drop detection on the printhead to check if any of the nozzles of the printhead are malfunctioning; (b) storing the result of the more recent drop detection operation, together with the results of the previous drop detections to keep a history of the health status of each nozzle: (c) deciding whether or not to execute a recovery service in the servicing area to attempt to recover the current malfunctioning nozzles, based on the more recent status of the nozzles and on the history of the health status of the nozzles.
By executing the servicing process only when it appears as necessary helps to improve the productivity of the printer and to reduce the waste of ink and to wear the nozzle plate.
Preferably, the method comprises the steps (d) of performing an evaluation of the history of the health status of a nozzle of the printhead and (e) of marking the nozzle as a recoverable nozzle or as an irrecoverable nozzle, according to the result of the evaluation and the step (f) of executing the recovery servicing if at least one recoverable nozzle, is detected as malfunctioning in the more recent drop detection.
In this way the servicing process is executed only if the malfunctioning nozzles are believed as recoverable. If all the failing nozzles are considered as irrecoverable, the process avoid to waist time and ink in running a likely useless service.
More preferably, the step (d) comprises the step of (g) keeping a score, representing the history of the health status of the nozzle, said score, determining the probability that said nozzle will not function, is increased each time that the nozzle is detected as malfunctioning or decreased when the nozzle is detected as working and wherein a recoverable nozzle is marked as an irrecoverable nozzle when the score of said nozzle reaches a first predetermined threshold.
Thus, it is provided a simple way of storing and comparing nozzle health information.
In a preferred embodiment, a step (h) of executing the recovery service is started if a number of recoverable nozzles bigger than a predetermined third threshold is detected as malfunctioning in the more recent drop detection.
This helps to avoid to run the servicing process when only few nozzles are detected as malfunctioning.
Typically, it is further comprised the steps (i) of performing a drop detection before executing a repeatable servicing procedure and, after having executed the repeatable servicing procedure on the printhead, (j) of comparing the result of the two drop detection steps in order to decide if the servicing procedure is to be repeated and the step (k) of repeating the repeatable servicing procedure if the percentage of recovered nozzles is bigger than a fourth predetermined threshold and a number of recoverable nozzles is still malfunctioning.
This helps to provide the pen with the minimum level of servicing as requested by the kind of defect of the pen and the more appropriate, in fact if the result of this specific servicing function is not good the process passes to the next level instead of repeating it.
In a further preferred embodiment the history of the health status comprises a sequence of historical values each value corresponding to the total number of malfunctioning nozzles as detected in a previous usable drop detection, and the recovery service is executed if the total number of malfunctioning nozzles is bigger than a fifth predetermined threshold, the total number of malfunctioning nozzles being selected from said sequence of historical values. In this case it is advantageously excluded the servicing in case of a non constant detection of the same high number of malfunctioning nozzles, i.e. if only a sporadic very negative drop detection occurs, no particular actions are taken to try to recover this abnormal lecture.
Preferably a usable drop detection is a drop detection performed before starting printing a plot, after having printed a plot or after having executed the recovery service. In this way it is advantageously excluded from the history all the drop detections occurred before the completion of all the servicing process, in fact those values may false the history of the nozzles adding a number of bad result and what is important to record is if the nozzles are responsive to the complete servicing strategy.
More preferably, said sequence of historical values is limited to the 8 usable drop detections more recently performed.
Viewing another aspect of the present invention, there is also provided a method of alleviating problems caused by malfunctioning nozzles of a printhead comprised in a pen mounted in an inkjet printing device, containing a drop detector, the method comprising the following steps performing a drop detection on the printhead to check the current health status of each nozzle of the printhead; based on the current status of the nozzles and on the history of the health status of the nozzles identifying the malfunctioning nozzles and deciding whether or not to start an improvement function to improve the quality of the output of the device.
Specific methods according to the present invention, recognize that by using a history of the nozzle health it is possible to improve the quality of the output in a number of different ways.
Preferably wherein the improvement function is selected from a group of functions including: a error hiding function, to attempt to minimise the usage of malfunctioning nozzles; a servicing function, to attempt to recover the malfunctioning nozzles; a printhead end-of-life function, to alert a user to replace the malfunctioning pen.
Viewing another aspect of the present invention, there is also provided inkjet printing device, for placing droplets of ink on a medium, comprising a pen comprising a printhead having a plurality of nozzles for ejecting droplets of ink, a droplet detector for identifying the nozzles of the printhead which currently present some malfunction in ejecting droplets of ink, said device being characterised by comprising a memory means for storing for each nozzle of the plurality of nozzles the history of the malfunction identified by performed droplet detections, said history being used by the device to alleviate problems caused by malfunctioning nozzles.
Preferably, the device further comprises a servicing means for recovering the defective nozzles, said history being used by the servicing means for selecting an appropriate servicing strategy.